Membrane based electrochemical cells, and particularly, proton exchange membrane (PEM) fuel cells are well known. PEM fuel cells convert chemical energy to electrical power with virtually no environmental emissions and differ from a battery in that energy is not stored, but derived from supplied fuel. Therefore, a fuel cell is not tied to a charge/discharge cycle and can maintain a specific power output as long as fuel is continuously supplied. The large investments into fuel cell research and commercialization indicate the technology has considerable potential in the marketplace. However, the high cost of fuel cells when compared to conventional power generation technology deters their widespread use. The cost of fabricating and assembling fuel cells can be significant, due to the materials and labor involved. Indeed, as much as 85% of a fuel cell's cost can be attributed to manufacturing.
A single cell PEM fuel cell consists of an anode and a cathode compartment separated by a thin, ionically conducting membrane. This catalyzed membrane, with or without gas diffusion layers, is often referred to as a membrane electrode assembly (“MEA”). Energy conversion begins when the reductants and oxidants, are supplied to the anode and cathode compartments, respectively, of the PEM fuel cell. Oxidants include pure oxygen, oxygen-containing gases, such as air, and halogens, such as chlorine. Reductants, also referred to herein as fuel, include hydrogen, natural gas, methane, ethane, propane, butane, formaldehyde, methanol, ethanol, alcohol blends and other hydrogen rich organics. At the anode, the reductant is oxidized to produce protons, which migrate across the membrane to the cathode. At the cathode, the protons react with the oxidant. The overall electrochemical redox (reduction/oxidation) reaction is spontaneous, and energy is released. Throughout this reaction, the PEM serves to prevent the reductant and oxidant from mixing and to allow ionic transport to occur.
Current state of the art fuel cell designs comprise more than a single cell, and in fact, generally combine several MEAs, flow fields and separator plates in a series to form a fuel cell “stack”; thereby providing higher voltages and the significant power outputs needed for most commercial applications. Flow fields allow for the distribution of the reactants through the fuel cell and are typically separate from the porous electrode-layers within the fuel cell. Depending on stack configuration, one or more separator plates may be utilized as part of the stack design to prevent mixing of the fuel oxidant and cooling input or exhaust streams within the fuel cell stack. Such separator plates also provide structural support to the stack.
Bipolar plates perform the same function as an oxidant flow field, fuel flow field and separator plate in combination and are often used in the design of fuel cells as their use can reduce the number of components required in the functioning fuel cell. These bipolar plates contain an array of channels formed in the surface of the plate contacting an MEA which function as the flow fields. The lands conduct current from the electrodes while the channels between the lands serve to distribute the reactants utilized by the fuel cell and facilitate removal of liquid reaction by-products, such as water. Fuel is distributed from the fuel inlet port to the fuel outlet port, as directed by the channels, on one face of the bipolar plate, while oxidant is distributed from the oxidant inlet port to the oxidant outlet port, as directed by the channels, on the opposing face of the bipolar plate, and the two faces are not connected through the plate. In the fuel cell stack, each bipolar plate serves to distribute fuel to one MEA of the stack through its fuel flow field face while distributing oxidant to a second MEA through the its opposite oxidant flow field face. The particular design of the bipolar plate flow field channels may be optimized for the operational parameters of the fuel cell stack, such as temperature, power output, and gas humidification. Ideal bipolar plates for use in fuel cell stacks are thin, lightweight; durable, highly conductive, corrosion resistant structures such as carbon/polymer composites, graphite or certain metals.
In the flow fields, the lands conduct current from the electrodes, while the grooves between the lands serve to evenly distribute the reactants utilized by a fuel cell, such as hydrogen, oxygen or air, over the faces of the electrodes. The channels formed by the lands and grooves also facilitate removal of liquid reaction byproducts, such as water. A thin sheet of porous paper, cloth or felt, usually made from graphite or carbon, may be positioned between each of the flow fields and the catalyzed faces of the MEA to support the MEA where it confronts grooves in the flow field to conduct current to the adjacent lands, and to aid in distributing reactants to the MEA. This thin sheet is normally termed a gas diffusion layer (“GDL”), and can be incorporated as part of the MEA.
Fuel cell stacks may also contain humidification channels within one or more of the coolant flow fields. These humidification channels provide a mechanism to humidify fuel and oxidants at a temperature as close as possible to the operating temperature of the fuel cell. This helps to prevent dehydration of the PEM as a high temperature differential between the gases entering the fuel cell and the temperature of the PEM causes water vapor to be transferred from the PEM to the fuel and oxidant streams.
Of necessity, certain stack components, such as the GDL portion of the MEA, are porous in order to provide for the distribution of reactants and byproducts into, out of, and within the fuel cell stack. Due to the porosity of elements within the stack, a means to prevent leakage of any liquid or gases between stack components (or outside of the stack) as well as to prevent drying out of the stack elements due to exposure to the environment is also needed. To this end, gaskets or other seals are usually provided between the surfaces of the MEA and other stack components and on portions of the stack periphery. These sealing means, whether composed of elastomeric or adhesive materials, are generally placed upon, fitted, formed or directly applied to the particular surfaces being sealed. These processes are labor intensive and not conducive to high volume manufacturing, thereby adding to the high cost of fuel cells. Additionally, the variability of these processes results in poor manufacturing yield and poor device reliability.
Fuel cell stacks range in design depending upon power output, cooling, and other technical requirements, but may utilize a multitude of MEAs, seals, flow fields and separator plates, in intricate assemblies that result in manufacturing difficulties and further increased fuel cell costs. These multitudes of individual components are typically assembled into one sole complex unit. The fuel cell stack is formed by compressing the unit, generally through the use of end plates and bolts, although banding or other methods may be used, such that the gaskets seal and the stack components are held tightly together to maintain electrical contact there between. These conventional means of applying compression add even more components and complexity to the stack and pose additional sealing requirements.
Other disadvantages observed in connection with some conventional fuel cell stacks are electrical in nature. For example, depending upon the configuration of the fuel cell and the degree of exposure of the MEA to the reactants and waste streams in the various manifolds providing reagents and coolants to the flow fields, a cross-cell potential problem may arise. In particular, if the exposure of the MEA to those reagents is significant, there may be a “shorting-out” of the MEA layer, thus resulting in poor performance of the fuel cell overall. Also, exposure of the MEA to some potential cooling fluids can be detrimental to the membrane portion. For example, in certain combinations of MEA and coolant, the coolant is capable of solvating or swelling the exposed portions of the membrane which can induce damage to the MEAs.
Various attempts have been made in the fuel cell art to address these deficiencies in fuel cell stack assembly design and thereby lower manufacturing costs. However, most require manual alignment of the components, active placement of the sealing means and/or a multi-step process.
Certain conventional processes are described in U.S. Pat. No. 6,080,503, to Schmid et al., U.S. Pat. No. 4,397,917, to Chi et al., U.S. Pat. No. 5,176,966, to Epp et al., and U.S. Pat. No. 5,264,299, to Krasij et al. However, notable disadvantages have been associated with those processes.
For example, U.S. Pat. No. 6,080,503, to Schmid et al. describes the replacement of gasket based seals within certain portions of the stack with an adhesive based material in the form of tapes, caulks or layers. However, assembly of that stack still requires manual alignment of the components during the adhesion process, in a manner not unlike caulking a seal, and sealing only occurs at those interfaces where adhesive has been applied through active placement.
Similarly, U.S. Pat. No. 4,397,917, to Chi et al., describes the fabrication of subunits within a fuel cell stack and is reported to provide ease in handling and testing. However, this design relies on conventional sealing among the components and between subunits. In addition, no manifolds internally penetrate the subunit.
See also, U.S. Pat. No. 5,176,966, to Epp et al., for its method of forming at least some of the required gaskets directly into the fuel cell stack assembly; and U.S. Pat. No. 5,264,299, to Krasij et al., which describes a fuel cell module having a PEM interposed between the two porous support layers which distribute reactant to the catalyst layers in which the peripheral portion of the support layers are sealed with an elastomeric material such that the PEM is joined with the support layers and the open pores of the support layers are filled with the elastomeric material making it fluid impermeable.
Additionally, World Publication WO 02/093672 describes a process for sealing fuel cell stacks via the injection of liquid resin. The reported process requires that all of the stack components be assembled first; then seals are introduced to produce the fuel cell stack. While that process may offer certain improvements to those previously described state of the art methods for forming fuel cell stacks, several deficiencies remain. In practice, for example, the process requires high injection pressures and is associated with slow fill times. High injection pressures further require component designs that protect the more fragile components of the stack (i.e. MEA). Another notable disadvantage is that a significantly large area of the each layer is necessarily sacrificed due to the sealing process itself.
Still further, in traditional fuel cell cassettes, two types of MEAs dominate; MEAs in which 1) the membrane extends beyond the borders of the gas diffusion layers, and 2) the gasket materials are formed into the edges of the MEA itself (with the membrane and GDLs approximately of the same size and shape, see for example U.S. Pat. No. 6,423,439 to Ballard). In the first type, separate gasket materials are used to seal between the membrane edge extending beyond the GDL and the other part of the stack (bipolar plates). In the second type, it is possible to seal directly to the other parts of the stack. Each of these methods requires compression to make a seal. These compressive-based seals require that all the components in the stack have high precision such that a uniform load is maintained. MEA suppliers have become accustomed to supplying the MEA formats discussed above.
In our previous patent applications, we have reported on an innovative fuel cell stack design which assembles together individual modules to form a fuel cell stack of requisite power output where each module permanently binds a number of unit cells together (see, World Publication WO 02/43173, the entirety of which is incorporated by reference).
Briefly, WO 02/43173 details a three-step process for the formation of fuel cell cassettes characterized by the following:
1) Sealing of unused manifold openings/ports on each of the particular flow fields (fuel, oxidant, and coolant). For example, in the case of the oxidant flow field, ports utilized for the distribution of fuel and coolant (on other layers) must be sealed about their perimeter to prevent the mixing of these input streams.
2) Sealing of all the ports within the membrane electrode assemblies (MEA) to prevent the leakage of the reactants within the MEA layers.
3) Layering these components (appropriately sealed as described) within a mold or fixture in a method prescribed by the particular stack design. Once the pieces are assembled within the fixture, a resin is introduced about the periphery. Using vacuum transfer molding or injection molding techniques, the resin is forced into the edges of the cassette assembly. Once hardened, it provides structural support and edge sealing over the assembly.
The resulting fuel cell cassette is then transformed into a fuel cell stack with the addition of end plates. Such a construction provides appropriate manifolding and a means of compression.
We also have developed innovative methods for sealing manifold ports within the stack or a module thereof, as well as methods for sealing the stack or module periphery that are less labor intensive and more suitable to high volume manufacturing processes (see World Publications WO 03/036747 and WO 03/092096, the entirety of which are incorporated by reference).
Despite even our own advancements in the field, it would be desirable to provide an improved fuel cell stack design that is less complex, more reliable, and less costly to manufacture. Additionally, it would be highly desirable to provide a method of making fuel cell cassettes utilizing roll-to-roll production of MEA, particularly, as this would greatly reduce the cost of this component. It also would be highly desirable to develop improved fuel cell cassettes which minimize or prevent exposure of the MEA to the reactants, waste streams, or cooling fluids around the various manifolds, thus avoiding cross-cell potential problems or material incompatibility associated with that exposure. Still further, it would be highly desirable to develop improved fuel cell stacks which can be formed at reduced injection pressures to simplify component design and which do not require that a significantly large area of the each layer to be sacrificed to accommodate the sealing process.